The invention relates to photo detectors, and particularly to ultraviolet (UV) light photo detectors. More particularly, the invention pertains to phototransistors.
GaN-AlGaN based solid state ultraviolet (UV) photo detectors, sensitive to 200 nanometer (nm) to 365 nm UV radiation, have been actively sought for applications including solar-blind UV detection and flame sensing. Due to the direct band gap and availability of Al.sub.x Ga.sub.1-x N in the entire alloy composition range (0&lt;x&lt;1), GaN-AlGaN based UV detectors have the advantages of high quantum efficiency, tunability of cut-off wavelengths, and the capability of being fabricated as heterostructures. In recent years, GaN-AlGaN photo conductors and photo diodes of both Schottky and PIN junctions with good performance have been reported. However, the need for internal gain in UV detectors remains for low light level applications, in which photodiodes often can not produce adequate signal levels for the read-out electronics. Some attention has been given to avalanche photodiodes (APD) but very limited success has been indicated. Typically a high leakage current is incurred in a GaN p-n junction before it reaches avalanche breakdown, which is most likely due to the high defect density in III-nitride materials.
Photo conductors fabricated on certain undoped and Mg-doped GaN layers have shown high photo current gains. Such gains can be attributed to trapped charges (e.g., holes), that have very small recombination cross sections and give rise to unusually long electron lifetimes. Consequently, these devices exhibit persistent photoconductivity (PPC) after UV exposure. Furthermore, PPC or dark current in a GaN photoconductor drifts with temperature which makes direct current type (DC) measurements very difficult. These characteristics of photo conductors render them useful only in low speed AC mode operations.
In addition to the above problems, the gains in GaN photo conductors are essentially defect-related effects (i.e., traps giving long carrier lifetime), therefore are sensitive to the nature and density of the defects formed during the material growth. This gives rise to difficulties in producing consistent detector characteristics due to the fact that defect formation in GaN is not well understood and controlled. It is thus desirable to obtain internal gain based on more controllable and better understood processes.